Genetic linkage

Genetic linkage occurs when particular alleles are inherited together. Typically, an organism can pass on an allele without regard to which allele was passed on for a different gene. This is because chromosomes are sorted randomly during meiosis. However, alleles that are on the same chromosome are more likely to be inherited together, and are said to be linked.

Because there is some crossing over of DNA when the chromosomes segregate, alleles on the same chromosome can be separated and go to different cells. There is much more chance of this happening if the alleles are far apart on the chromosome, as it is more likely that a cross-over will occur between them.

The physical distance between two genes can be calculated using the offspring of an organism showing two linked genetic traits, and finding the percentage of children where the two traits don't run together. The higher the percentage of offspring that don't show both traits, the further apart on the chromosome they are.

A study of the linkages between many genes enables the creation of a linkage map.

Two phenotypes (height and texture) occur randomly with respect to one another in a manner known as independent assortment. Today scientists understand that independent assortment occurs when the genes affecting the phenotypes are found on different chromosomes.

An exception to independent assortment develops when genes appear near one another on the same chromosome. When genes occur on the same chromosome, they are inherited as a single unit. Genes inherited in this way are said to be linked. For example, in fruit flies the genes affecting eye color and wing length are inherited together because they appear on the same chromosome.

But in many cases, genes on the same chromosome that are inherited together produce offspring with unexpected allele combinations. This results from a process called crossing over. Sometimes at the beginning of meiosis, a chromosome pair (made up of a chromosome from the mother and a chromosome from the father) may intertwine and exchange sections of chromosome. The pair then breaks apart to form two chromosomes with a new combination of genes that differs from the combination supplied by the parents. Through this process of recombining genes, organisms can produce offspring with new combinations of maternal and paternal traits that may contribute to or enhance survival.

Genetic linkage was first discovered by the British geneticists William Bateson and Reginald Punnett shortly after Mendel's laws were rediscovered.

Linkage mapping

The observation by Thomas Hunt Morgan that the amount of crossing over between linked genes differs, led to the idea that crossover frequency might indicate the distance separating genes on the chromosome. Morgans student Alfred Sturtevant, developed the first genetic map, also called a linkage map.

Sturtevant proposed that the greater the distance between linked genes, the greater the chance that non-sister chromatids would cross over in the region between the genes. So by working out the number of recombinants it would be possible to obtain a measure to map the distance between the genes. This distance is called a genetic map unit (m.u.), or a centimorgan and is defined as the distance between genes for which one product of meiosis in 100 is recombinant. A recombinant frequency (RF) of 1 percent is equivalent to 1 m.u.

See also: Recombinant frequency

Lod score method for linkage

The Lod (log of odds) score is used to calculate the the probability of a pedigree arrising randomly or by genetic linkage. The test was developed by Newton E. Morton.

<math> LOD = log \frac{probability\ of \ birth\ sequence\ with\ a\ given\ linkage\ value}{probability\ of\ birth\ sequence\ with\ no\ linkage}<math>fr:Lod score